Picture yourself as one of England’s majestic Bewick’s swans, about to take off on your annual long-distance flight to Arctic Russia, when out of nowhere a scientist grabs you and methodically gropes and measures your butt. It’s all for your own good: Researchers are hurriedly sizing up as many round rumps as they can lay their hands on, in a bid to understand what’s wiping out their population.

Smaller than the more common mute swans, which stay in Britain yearlong, Bewick’s swan has seen its population in Europe decline from 29,000 to 21,000 between 1995 and 2005, and researchers at UK’s Wildfowl and Wetlands Trust (WWT) in Slimbridge, Gloucestershire, are willing to fondle the birds to save them.

They’re sizing up swans to test whether changes in the their habitat are to blame for their decline: The size of swan keesters indicated whether they have enough fat to survive their over-2,000-mile journey. Basically, if the birds are plump, then that rules out the possibility that they aren’t getting enough food, and opens the playing field for other culprits, such as power line collisions, lead poisoning, and hunting.For those of you without much swan-butt-measuring experience, here’s a rough guide: As WWT researcher Julia Newth told the BBC, “In a slim bird, the bum will look slightly concave, whereas a well-fed bird will have a double bulge.” But the scientists don’t merely ogle, they actually measure the area between the two legs and the tail—the precise spot where swans build fat deposits for their migrations.

Even though the study is still in progress, some preliminary findings have already surfaced: Most of the birds already measured have, as Newth puts it, “big healthy behinds.” And has been well established from previous research, big butts (and their likers) cannot lie, suggesting that some other factor is to blame. But the researchers will have to wait several months, till the swans return, to undertake any further tests.